CLASSIFICATION AND ADAPTATION 33 



found for it. It is a generic character, so far as we 

 know, without utility. On the other hand, it is very 

 probable that this subcaudal extension of the fins 

 is merely a result of the posterior extension and 

 enlargement of these fins which has taken place in 

 the evolution of the adaptation. If the Lamarckian 

 explanation of adaptation were true, it would be 

 possible to understand that the constant movements 

 of the fins and muscles by which the adhesion was 

 effected caused a longitudinal growth of the fins 

 in excess of the length actually required, and that 

 this extra growth extended on to the body beneath 

 the tail, although the small flaps on the lower side 

 were not necessary to the new function which the 

 fins performed. 



When we consider such cases as this we are led 

 to the conclusion that the usual conception of adap- 

 tation is not adequate. We require something more 

 than function or utility to express the difference 

 between the two kinds of characters to be distin- 

 guished. For example, the absence of pigmen- 

 tation from the lower sides of Flat-fishes may have 

 no utility whatever, but we see that it differs from 

 the specific markings of the upper side in the fact 

 that it shows a relation to or correspondence with 

 a difference of external conditions namely, the 

 incidence of light, while in such a case as the red 

 spots of the Plaice we can discover no such corres- 

 pondence. 



We know that the American artist and naturalist 

 Thayer has shown that the lighter colour of the 

 ventral side of birds and other animals aids greatly 

 in reducing their visibility in their natural surround- 

 ings, the diminution in coloration compensating for 



c 



